This invention relates to an optical device such as an optical switch, optical isolator or optical circulator and, more particularly, to a polarization-independent, high-isolation optical device that uses a novel thin film polarizing beam-splitter.
In fiber telecommunications, and in particular in wavelength division multiplexing, there is a need for high performance, low-cost and easily-producible optical switches, isolators and circulators. Optical switches are used to select fiber channels electronically. Optical isolators are commonly used in optical amplifiers that amplify fiber signals without using repeating stations. These optical amplifiers are pumped by diode lasers, which are very sensitive to any light reflected back to their cavities. Optical isolators can be used to isolate any reflected light going back to the lasers.
Recently, optical circulators have become very important in bi-directional fiber communications. In a multi-port circulator, signals go from port 1 to port 2, port 2 to port 3, port 3 to port 4, and so on, in stead of port 1 to port 2 and port 2 to port 1. For example, in Bragg grating wavelength division multiplexers (WDM), without using a circulator, the reflected signal would come out from the same port that the incident light goes in; as result, the incident light and the reflected light cannot be physically separated. However, if a three-port circulator is used, the reflected light will come out from a different port. In addition, optical circulators are also used in channel dropping and adding from and to main fiber lines.
Typically, an optical device such as an optical switch, isolator or circulator has a similar structure. It includes a polarization-rotating device sandwiched between two polarizing devices. The first polarizing device is used to separate the incident beam into two orthogonal polarized light beams and the second polarizing device is used to combine the two orthogonal polarized light beams into one output beam. For a polarization dependent optical switch isolator or circulator, only one polarized light is used. The polarization-rotating device normally consists of a reciprocal device or a non-reciprocal device, or the combination thereof.
A typical reciprocal device is a waveplate such as a quarterwave plate or halfwave plate. A quarterwave plate changes a linear polarized light into a circular polarized light if its optical axis is aligned 45xc2x0 with regard to the polarization of the incident linear polarized beam. A halfwave plate rotates the polarization of a linear polarized light by any angle depending on the alignment of its optical axis with regard to the polarization of the incident beam.
A typical non-reciprocal device is a Faraday rotator. When a magnetic field is applied to the Faraday rotator, it rotates the electric field of a linear polarized light by a certain angle. The rotational angle depends on the property and the length of the Faraday rotator as well as the strength of the magnetic field. The direction of the rotation depends on the direction of the magnetic field. Therefore, the polarization plane of the light beam is rotated in the same direction for light coming from both directions. This is why such a device is called non-reciprocal device.
Normally, such an optical device has several input and output ports. For an optical switch, the output beam is switched between the several output ports electronically. In order to do this, a mechanism is applied to alter the direction of the magnetic field, for example, an electric coil can be used in which the current can be switched on in both directions. For an optical isolator, the light comes in reverse direction is not used and is directed to a port that is different from the incident port. To use as an optical circulator, the signals circulate between all the ports.
Currently, optical switches, isolators and circulators are mainly based on birefringent polarizing devices such as birefringent polarizers, wedge polarizers or walk-off polarizers, for example, U.S. Pat. Nos. 5,446,578 and 5,734,763 by Chang and U.S. Pat. Nos. 5,581,640, 5,566,259, 5,557,692, 5,706,371 by Pan et al. Sometimes absorbing plate polarizers are also used in optical devices which are polarization-dependent.
Although birefringent polarizers have the advantage of having high extinction ratios, there are several disadvantages resulting from their use. First, birefringent polarizers are expensive. Second, these polarizers have birefringent effects that result in polarization mode dispersion. In order to overcome this problem, other birefringent plates or a second identical stage are added to compensate this polarization dispersion. Both approaches require the use of more birefringent plates or polarizers, and this makes it very expensive and very difficult to assemble since the optical axes of all the birefringent elements need to be accurately aligned. Third, the most common configuration in conventional optical isolators or circulators uses walk-off birefringent polarizers to separate ordinary (o) and extra-ordinary (e) rays physically. This separation depends on the refractive index difference between o- and e-rays and the size of the birefringent material. The greater the separation, the easier it is to package and the better the performance. However, since the refractive index differences depend on the available birefringent materials which are limited, so an increase in the separation means an increase in the size of the birefringent plate. As a result, it is more expensive because the greater the size, the more expensive the birefringent materials. Fourth, it is difficult to make an N multi-port optical circulator based on birefringent materials with the number of ports N larger than four.
Conventional thin film polarizing devices such as thin film polarizers or thin film polarizing beam-splitters (PBS), including MacNeille polarizers or thin film cube or plate polarizers, have been proposed for use as polarizing devices in optical switches, isolators and circulators. For example, one example of the optical circulator was described in U.S. Pat. No. 4,272,159 by Matsumoto. The thin film interference polarizers and PBSs consist of multilayers of dielectric films deposited onto glass or other substrates. Such polarizers reflect s-polarized light and transmit p-polarized light and are normally based on the light interference in thin films, sometimes also in combination with other effects.
Although conventional thin film polarizing devices are versatile in terms of design and are not limited by size and are easier to make and hence less expensive, one of their biggest disadvantages is the low extinction ratio (less than 30 dB isolation), especially in the reflected beams. In addition, the bandwidth of the thin film cube or plate polarizers is very small. Another disadvantage is that their angular field is very small, and they therefore require well collimated light beams. As a result, any optical device based on these conventional thin film polarizing devices will suffer the same low extinction ratio problem. In addition, they are more difficult to package because of the small angular fields. Such optical switches isolators and circulators can only be used in the areas where high extinction ratios are not required. For high performance devices, such as those used in fiber communications, the market is dominated by the birefringent materials.
The most commonly used thin film polarizers are the MacNeille polarizer which was invented by MacNeille in 1946. It is based on the Brewster angle phenomenon and light interference in thin films. When light is incident at the interface between a high and low refractive index materials, if the incident angle is equal to the Brewster angle, all the p-polarized light is transmitted and s-polarized light is partially reflected. In order to increase the reflection for s-polarized light, a multilayer interference coating consisting of the high and low index materials are used. The coating is sandwiched between two glass prisms, which is required by the Brewster angle requirement. The multilayer coating acts as a high reflector for the s-polarized light and does not affect the transmission of the p-polarized light at the Brewster angle. The reflection band for s-polarized light depends on the refractive index ratio of the high and low index materials and can be extended by chirping the layer thickness or by using several layer stacks. Hence, the MacNeille polarizer is broad band; however, it is very sensitive to the variation of the angles of incidence. Once the incident angle moves away from the Brewster angle (xc2x12xc2x0), the performance of the polarizer deteriorates dramatically. In addition, the extinction ratio for the reflected beam is low because the index-mismatch between the prism substrate and the coating materials.
Another thin film PBS (polarizing beam splitter) is based on the edge separation between s- and p-polarized light of an edge filter at an oblique angle of incidence. In this separation region, s-polarized light is reflected and p-polarized light is transmitted. Its angular field is relatively large compared to MacNeille polarizer. The extinction ratio of such polarizer can be very high in the transmitted beam if a large number of layers are used to reflecting s-polarized light. However, a high extinction ratio can not be achieved for the reflected beam. In addition, such a polarizer has a very small bandwidth. As a result, it is often used for narrow band applications such as lasers.
It is therefore an object of the present invention to provide a low-cost, high isolation and polarization-independent optical device that can be used as an optical switch, isolator or circulator.
In its most general aspect the invention provides an optical device for controlling the flow of light between ports, comprising a pair of thin film polarizing devices, the improvement wherein said thin film polarizing devices employ frustrated total internal reflection and interference in a thin film coating to transmit s-polarized light and to reflect p-polarized light.
It will be understood by one skilled in the art that a thin film coating typically consists of multilayers formed on a substrate.
Typical thin film polarizing devices are polarizers or polarizing beam splitters (PBS). It will be understood that depending of the direction of light, such polarizing devices can be used to split unpolarized light into separate s- and p- polarized beams or to combine such separately polarized beams into a single unpolarized beam. The term polarizing device in this specification covers such devices whether functioning as beam splitters or beam combiners. Several parameters that are used to describe the performance of a polarizing device are:
1. the wavelength range, which is the range over which the polarizing device is effective;
2. the angular field, which is the angular field of the incident light in which the polarizing device is effective;
3. the extinction ratio, which is the ratio of the desired polarized light to the unwanted polarized light after the light passes through or is reflected from the polarizing device; and,
4. the transmittance or reflectance for the desired polarization.
Polarizing devices employed in the invention are non-absorbing, and have broadband wavelengths, wide angular fields and high extinction ratios, also are easier and less expensive to manufacture. In a typical application, one polarizing device functions as a beam splitter to split incident unpolarized light into separate s- and p- polarized beams and the other polarizing device functions to combine the beams into a single unpolarized beam. A polarization-rotating device, which may be either reciprocal or non-reciprocal, may be placed in the respective p- and s- polarized beams. Such an arrangement can be used to make multi-port optical switches, isolators or circulators.
In a preferred embodiment a first of the polarizing devices splits a light beam incident at a first port into a reflected p-polarized beam and a transmitted s-polarized beam, and a second of said polarizing devices combines a p-polarized beam and a s-polarized into a combined unpolarized output beam at a second port. A polarization-rotating device, such as a Faraday rotator, can be inserted in the beams to control the flow of light between the ports and thus create optical switches, isolators or circulators. Such devices do not have polarization mode dispersion if a symmetrical configuration is used. The insertion loss in these devices can be small as well. The optical device can also be made polarization dependent, in which case only one polarized beam is used. A single polarizing device directs incident polarized light through the input port of a polarization-rotating device to a reflecting surface, from where it is reflected back into the polarizing device, with its plane of polarization changed. The reflected beam appears at an output port.